Speed humps slow drivers down thus making accidents less likely and
less severe.

Not necessarily - they slow down for each hump and speed up in between - so
they may well be going faster in between the humps than they would be if the
humps weren't there.

As for speed limit in force at all times well if it wasnmt drivers would
get
confused, and is travelling at 20 over that distance such a problem??.

But as it's the school run parents who are usually the ones responsible for
speeding outside schools, surely having a 24/7 20-limit is wasteful and
unnecessary?